Safe, convenient and economic disposal of waste products, particularly biological wastes, remains a problem for society. As municipal authorities become increasingly aware of their responsibilities in this area, legislation usually requires commercial enterprises to ensure that safe disposal occurs of wastes resultant from their activities. As an incentive for a commercial enterprise to undertake the necessary treatment prior to disposal of its waste products, financial penalties are levied by municipal authorities unless the treated waste meets certain requirements before discharge into the public sewer system.
Some waste materials of interest include piggery (and other animal) wastes, brewery wastes, cattle feedlots (beef and dairy), domestic activities inclusive of sewage, fruit Juicing, canning, abattoirs, dairy and cheese factories, alcohol distilleries, soft drink factories and food processing.
The basic thesis underlying the treatment of biological wastes is that the biochemistry and microbiology of each waste, and then that of the proposed treatment process, must be understood in order for the treatment to work in the desired manner. It is precisely because an understanding of these fundamentals has been lacking, that a comprehensive treatment of wastes inclusive of biological waste has yet to be proposed.
Pig wastes for example consist of a solid portion made up of faecal matter and spilt feed, and a liquid portion consisting of urine, water lost from the drinking fountains, wash-water used in cleaning the piggery and a small quantity of material infused from the solid portion. The solid material contains two fractions one of which is easily digested and represents 36-40% of the total solids (ie Hobson and Robertson (1977) in Applied Science Publishers Ltd London entitled "Waste Treatment in Agriculture") while the other fraction is of low digestibility. The digestible fraction contains proteins, fats and carbohydrates, organic acids and breakdown products of nitrogenous compounds while the relatively indigestible fraction is largely lignocellulose. The two fractions in the solid portion are closely interlinked with each other. The liquid portion is rich in salts derived from the urine and contains nitrogen, both as urea and as ammonia, and elements in high concentration such as calcium and phosphorus as well as potassium, sodium, iron, copper, zinc, sulphur and chloride. Both the solid and the liquid portions harbour a rich flora of micro-organisms in which it would be unusual not to find some pathogens. The solid faecal portion is rich in iron, copper and zinc which are provided as additional supplements to the pigs for nutritional reasons. The entire waste has a unique and, to most people, unpleasant odour.
If it assumed that the mean body-weight of a pig in a piggery producing bacon is 47 kg, then such a pig voids daily 0.263 kg of faeces and urine as drymatter and around 2.5 liters of water in urine. If it is further assumed that a herd of 1000 sows would contain a mean population of 9050 pigs at any one time, then the daily output of faeces and urine (drymatter) would be 2.38 tonnes and the output of urine as water would be 22,626 liters. These data are used merely to outline in general the size of the daily waste output of a piggery. When combined with wash-water the output of waste is formidable, and a piggery of the size just postulated could produce 200,000 liters of liquid waste per day.
It therefore will be appreciated that the opportunities which would arise from a treatment process which would deal adequately with piggery and other biological wastes are considerable and could be marketed on a global scale.
The implications of development of such a treatment process, if successful, are that pig farming would no longer be regarded as a nuisance industry, a danger to public health and a despoiler of the environment.
In relation to successful treatment of piggery waste, for example useful criteria that may be taken into account for a successful disposal of such waste may include the following:
1. That the unpleasant odour associated with piggery waste--both solid and liquid--be removed. PA0 2. That pathogens such as Salmonella, Escherichia coli, Campylobacter and Treponema be destroyed. PA0 3. That water derived from the treated waste be capable of reuse, at least as wash-water, and be suitable and safe for disposal to land if so required. This implies that the pH of the treated water be around neutrality (eg. in the range 6.5-7.5). PA0 4. That the indigestible lignocellulosic sludge be stabilised and made safe for disposal to land. PA0 5. That part of the waste be converted to a product which has some economic value. PA0 6. That the total oxygen demand (TOD) in the waste be reduced significantly. PA0 7. That preferably the nitrogen in the waste water be reduced significantly. PA0 8. That preferably the phosphorus in the waste be reduced significantly eg. by at least 30% of its original concentration. PA0 9. That the production of greenhouse gases--methane and carbon dioxide--be minimised by absorbing a significant amount of carbon in the waste into microbial cells. PA0 10. That preferably the residence time of the water undergoing treatment in the process be of relatively short duration eg not to exceed 8 days. PA0 11. That some or all of the preceding criteria be realised by the treatment, and the capital and operating costs be less than that of any other comparable process. PA0 1. The waste is fermented semi-continuously (feeding the fermenter once daily) for a 5-day residence time. (A 6-day residence did not increase the concentration of .organic acid). This inhibits the formation of methane and miniwises the formation of carbon dioxide so that most of the carbon is directed into organic acid compounds. PA0 2. The fermented liquor is held for two days at pH 4.6 or less in another vessel to settle the undigested solids (lignocellulose) and destroy the pathogens. PA0 3. The fermented liquor, freed of bacterial pathogens, is then treated aerobically by Candida inqens to remove the unpleasant odour, most of the carbon present as organic acids and some of the nitrogen and phosphorus.
It will be appreciated that the above criteria may also be modified. substantially when dealing, with successful disposal of other biological waste (eg brewery waste or distillery waste).
It has also been shown in Henry Aust. Vet. J 51, 317-319 (1975), Henry et al Appl. and Environ Microbiol 31, 813-818 (1976), Henry et al Search , 161-163 (1976), Henry et al Appl and Environ Microbiol 1132-1136 (1979) and Henry et al J. Appl. Bacteriol 55, 89-95 (1983) that working with piggery wastes showed that when the liquid and solid wastes (10% solids) are combined in a closed vessel and maintained at a temperature of 37.degree. C. and fed once daily on a semi-continuous regime with a 5-day residence time, a fermentation takes place and the digestible parts of the solids are converted mostly to organic acids and carbon dioxide, although some of the resultant compounds have a nitrogenous component. The fermentation is effected by the anaerobic bacteria present in the waste which has been voided by the pigs.
There is virtually no methane or hydrogen in the headspace of the bioreactor. The proteins, fats and carbohydrates in the waste are thus converted by the fermentative and acid-forming organisms to relatively simple chemical compounds (eg. acetic acid). The result is a separation of the digestible and indigestible parts of the solid portion of the piggery waste. The digestible part of the solids thus becomes part of the liquid portion of the waste which has become, as a result, rich in organic acids. This separation must take place if the criteria for successful treatment, which have been listed, are to be realised.
The semi-continuous fermentation proceeds at a pH of 5.8-6.0. The combination of acid pH and short residence time enables much of the carbon liberated by the fermentation to be incorporated into organic acid. concentrations of 200 mM (range 170-210) of volatile fatty acids are obtained. At the pH of the fermentation the VFA present in the waste are in the form of acid salts eg. ammonium acetate, propionate, butyrate, valerate and caproate. If the pH is lowered to -4.6 or less with mineral acid (eg. sulphuric acid), about 85% or the acid salts are converted to free acid (eg. acetic acid, propionic acid etc.). Free VFA will kill pathogenic bacteria such as Salmonella and Escherichia coli, and Henry et al Journal Appl. Bact. 55: 89-95 (1983) demonstrated that total destruction of these organisms could be effected in piggery waste. The concentrations of VFA repaired to kill salmonella and E. coli are 30 mM and 90 mM respectively at pH 4.0 and 30.degree. C. The fermented liquor is held for two days at pH 4.0 and 30.degree. C. to kill the pathogens and this also enables clarification of the fermented liquor to occur. The settled sludge consists of lignocellulose from the faeces and this is well stabilised (ie. it is not easily fermentable any further and it is odourless when wet or dry). In subsequent work Henry and Cossins (1982 unpublished) found that this treatment removed Vibrio cholerae from an aqueous medium and Henry in 1988 (unpublished) found that three species of Campylobacter would not survive this treatment. It is expected that treponemes would also be eliminated as they are much more fragile organisms than the enterobacteriaciae (eg. Salmonella), but this has not yet been proved by experiment.
Henry as established by the abovementioned Appl. and Environ. Microbiol. 1976 and 1979 references then showed that a yeast, Candida ingens, growing as a pellicle on fermented piggery waste could remove almost all (95%) of the organic acids--volatile and non-volatile--and another nitrogenous compound, methylindole (skatole) (Henry unpublished) a breakdown product with an unpleasant odour, of the amino acid tryptophan. It was found that C. ingens also removed much of the ammonia nitrogen, and phosphorus. Mineral analysis of C. inqens demonstrated that the organism growing as a pellicle on fermented piggery waste contained phosphorus as 7% of the dry matter (Henry et al Appl. Environ above 1976). It was shown much later by Henry (1984 unpublished) when working with alcohol distillery wastes that C. inqens could metabolise many of the amino acids. A notable fact arising from growing C. inqens, as a pellicle, on fermented piggery waste is that the yeast removes the unpleasant odour from the waste. The only odour in the spent liquor remaining after treatment with C. ingens is a faint mustiness.
Features of the pellicle process are:
The yeast derived from the treatment is suitable as a feed for monogastric animals such as pigs, poultry and domestic pets. The spent liquor is suitable for use as wash water or for irrigation.
While work on the pellicle process was approaching completion, and commercialisation was being contemplated, the pig industry underwent a rapid change in structure and a number of very large units were established. As sunlight damaged C. ingens, the shallow ponds in which C. ingens grew on fermented pig waste had to be covered. In these large units the volume of liquid to be accommodated in covered ponds was so great that the process became of doubtful economic value.
At that time the only other technologies using aerobic processes to effect rapid treatment of liquid wastes were submerged culture in a bioreactor or aeration using a motor-driven paddle aerator. Neither of these technologies met the criteria for successful treatment of piggery waste described above and therefore were found to be deficient.
For example, the production of single cell protein (SCP) by the aerobic surface of culture of yeasts or biological wastes has been disclosed, inter alia, in Australian Patent No. 475824 and U.S. Pat. No. 4,115,593.
Similarly, although production of SCP from liquid wastes using yeasts in submerged culture has been tested, it has not, as yet, reached commercialisation, principally because, while submerged culture has advantages over the surface method, the equipment purchase, operational and maintenance costs weigh against its adoption.
Similar problems as described above in relation to disposal of piggery wastes were encountered in regard to disposal of other wastes in for example the brewing industry and also in distilleries such as rum distilleries where effective disposal of dunder (ie cane juice or waste) is relatively difficult. Thus for example in relation to brewery wastes such wastes are deficient in some nutrients that are essential for microbial growth and thus such nutrients which include nitrogen sulphur and phosphorus have to be added as crude agricultural chemicals before biological waste treatment can be carried out. This in turn created problems in pH reactions. In relation to disposal of dunder this is a high strength waste and it has been ascertained that the efficiency of removal of nutrients from a waste by microorganisms is inversely related to the concentration of nutrients in the waste. An extreme illustration of this principle is the complete inhibition of the growth of micro-organisms by high concentrations of sugar or salt in solution. Dunder with a TOD of about 100,000 mg per liter contains extremely high concentrations of a wide variety of organic compounds and some minerals such as potassium and this is a major factor which so far has prevented effective biological treatment of such waste. This factor also applies to successful treatment of brewery waste which has a TOD of 60,000 mg per liter.
Other techniques that have been utilised so far in treatment and disposal of biological wastes and which have proved deficient for similar reasons as discussed above include the following:
(i) a combination of anaerobic and aerobic treatments which are included in conventional "activated sludge" plant.
(ii) anaerobic ponds and septic tanks which are a modified form of anaerobic ponds; and
(iii) pits included in piggery floors wherein piggery effluent is passed to these pits and mixed by a paddle wheel or alternatively a system where slits located in a piggery floor open out into a tear drop shaped pit which is connected to a drain for disposal of effluent.
Reference may also be made to trickling filters which usually comprise a plurality of rocks or other suitable particular matter wherein waste of a relatively low TOD value (ie between 10-100 mg per liter) may be trickled through the rocks whereby after a period of time a bacterial film will form on the rocks under the influence of air which also may be blown through the rocks. This will result in oxidation of compounds in the waste. However one basic disadvantage of this process was that often great difficulty was achieved in sloughing the bacterial film of the rocks after a period of time. This meant that often the bacterial film was anaerobic in nature which meant that the compounds in the waste could not be oxidised. Also this process like all other conventional processes could not be used with biological wastes of high TOD value such as distillery waste, brewery waste or piggery waste described above.
It therefore is the object of the present invention to provide a process and apparatus for effective treatment of biological wastes which may alleviate the problems discussed above in relation to conventional methods of biological waste treatment.